THALLOPHYTES : ALG.E 231 
The gametes are organized within a mother cell, and if 
this cell is distinct from the other cells of the plant it is 
called a gametangium, which means " gamete vessel." 
This method of reproduction may be indicated by a for- 
mula as follows : P = > o P = > o P = > o P, 
which indicates that two special cells (gametes) are pro- 
duced by the plant, that these two fuse to form one (sexual 
spore), which then produces a new plant. 
At first the two gametes are alike in size and activity, 
and such plants are said to be isogamous that is, " with 
similar gametes." In other plants the gametes become 
very dissimilar, one being large and passive, and called the 
egg ; the other being small and active, and called the 
sperm ; and such plants are said to be heterogamous that 
is, "with dissimilar gametes." The gametangium which 
produces the egg is called an oogonium; that which pro- 
duces sperms is the antheridium. 
It must not be supposed that if a plant uses one of these 
three methods of reproduction (vegetative multiplication, 
asexual spores, sexual spores) it does not employ the other 
two. All three methods may be employed by the same 
plant, so that new plants may arise from it in three diifer- 
ent ways. 
